The present invention is related to fabrication of circuit boards and microelectronic devices containing signal lines embedded in dielectric substrates and, in particular, to circuit boards and microelectronic devices in which dielectric material surrounding striplines is eliminated, effectively substituting air for the dielectric material, in order to greatly decrease the dielectric loss coefficient for the stripline.
In circuit boards and in various microelectronic devices, numerous signals paths electrically interconnect electrical components and subcomponents affixed to, or incorporated within, the circuit boards and microelectronic devices. In many cases, these signal lines, also called xe2x80x9cstriplinesxe2x80x9d and xe2x80x9ctraces,xe2x80x9d are composed of copper, or another conductive element or alloy, embedded within a dielectric substrate, such as a fiberglass and epoxy composite or a plastic material. In general, the signals transmitted through signal lines represent information as discrete voltage pulses, with a relatively low voltage state representing a first binary value and a relatively high voltage state representing a second binary value. When the rise/fall time of the signal within the signal line is fast enough that the signal line can change from one logic state to another in less time than it takes for the signal to traverse the length of the signal line, then the signal line is typically mathematically modeled as a transmission line. In a transmission line, the strength, or intensity, of the signal is attenuated as the signal traverses the signal line. The attenuation of the signal a is modeled as the sum of two attenuation coefficients:
xcex1=xcex1C+xcex1D
where xcex1C=attenuation coefficient for conduction, and
xcex1D=attenuation coefficient for dielectric loss
The attenuation coefficient xcex1is normally expressed in dimensions of decibels/meter (xe2x80x9cdB/mxe2x80x9d). The attenuation coefficient xcex1C arises from resistance of the conductive element or alloy from which the signal line is composed, and the attenuation coefficient xcex1D arises from dissipation of energy within the dielectric substrate surrounding the signal line. This dissipation of energy occurs as molecules within the dielectric substrate, having either permanent or induced electric dipole moments, realign themselves within the fluctuating electric field produced by the electric signal transmitted through the signal line. The attenuation coefficient xcex1D is proportional to the frequency of the signal, as shown by:       α    D    ∝            π      ⁢                        ϵ          r                    ⁢      f      ⁢              xe2x80x83            ⁢      tan      ⁢              xe2x80x83            ⁢      δ        c  
where xcex5R is the relative permittivity,
tan xcex4is the loss tangent,
f is signal frequency, and
c is speed of light
The relative permittivity and the loss tangent are characteristic for each different dielectric material. The relative permittivities and loss tangents for a number of materials are provided below, in Table 1:
FIG. 1 shows a graph of the attenuation of signal intensity per unit length versus signal frequency. In FIG. 1, the vertical axis 102 represents the relative signal intensity following transmission of the signal through a unit length of signal line, and the horizontal axis 104 represents the logarithm of the signal frequency. Curve 106 represents theoretical signal intensity attenuation due to dielectric loss, or xcex1D and curve 108 represents signal attenuation due to resistivity of the conductive element or alloy from which the signal line is composed. Curve 110 is the calculated overall signal attenuation, and curve 112 is an experimentally measured signal attenuation. Note the steep increase in signal attenuation in the gigahertz signal frequency range.
FIG. 2 illustrates the effects of frequency-dependent attenuation of signal strength within a signal line on the signal transmitted within the signal line. FIG. 2 shows a desirable low-state-to-high-state signal transition 202 and a low-state-to-high-state signal transition 204 when significant signal intensity attenuation occurs due to resistivity of the conducting element or alloy and to dielectric loss. Both low-state-to-high-state transitions are plotted in time against a common time (t) axis 206 and against separate amplitude (xe2x80x9cAxe2x80x9d) axes 208 and 210, with the vertically upward direction of increasing amplitude indicated by the vertical arrow in FIG. 2. For the desirable transition 202, the signal rises from a low state 212 to a high state 214 within a relatively short period of time xcex94t 216. For the desirable transition, the rise xcex94A 218 corresponds to the amplitude or voltage differential generated at the source of the signal. However, when signal attenuation occurs, as described above, the transition time xcex94t 220 may increase and the amplitude or voltage differential 222 may decrease. This lower final amplitude is due to conduction losses.
Signal intensity attenuation within a signal line may lead to inoperability of an electronic circuit containing the signal line. Attenuation of the signal intensity may prevent the signal from rising above a voltage or amplitude differential threshold required for signal detection by the destination component or subcomponent connected to the signal line. Increase in the time of transition between low and high voltage or amplitude states may prevent transmission of the signal altogether. As discussed above, with reference to FIG. 1, the degradation of transition times and signal intensity is frequency dependent, and greatly increases in the gigahertz range. However, modern microprocessors are currently operated at frequencies in the gigahertz range, and are continuously being enhanced to operate at faster speeds. Transmission of signals between microprocessors and other electronic components within circuit boards and microelectronic devices has become a serious bottleneck constraining overall circuit-board and microelectronic-device processing throughput and speed of operation.
FIG. 3 shows a small section of a circuit board or microelectronic device including two embedded signal lines. In this portion of a circuit board or microelectronic device, the two conductive signal lines 301 and 302, also called xe2x80x9cstriplines,xe2x80x9d or xe2x80x9ctraces,xe2x80x9d are embedded in a dielectric material 303 parallel to two conductive planes 304 and 305. The conductive planes 304 and 305 serve as electrical reference planes, or ground planes, for the signal lines 301 and 302. According to the above discussion, for the traces 301 and 302 to support signal transmission, the signal intensity attenuation must be maintained below a threshold value that depends on the signal response characteristics of subcomponents interconnected by the signal lines and by required times for signal state transitions. As discussed above, signal intensity attenuation is frequency dependent, so as the frequencies of signals carried by the signal lines increases with increasing microprocessor speed, circuit board and microelectronic device designers must more and more carefully control design and material parameters in order to maintain signal intensity attenuation below necessary threshold values.
Currently, circuit board and microelectronic device designers maintain signal intensity attenuation below threshold values by either minimizing the length of signal lines, choosing materials for signal lines having low resistivities, or by choosing dielectric substrate material with low relative permittivities and, most particularly, with low loss tangents. Unfortunately, substrate materials generally increase in cost with decreasing loss tangents. For example, polytetrafluoroethylene (xe2x80x9cPTFExe2x80x9d) has a loss tangent several orders of magnitude below that of FR4 fiberglass, but is far more expensive as a bulk material, and may additionally increase manufacturing costs due to changes in manufacturing procedures required for PTFE-based fabrication. Designers and manufacturers of circuit boards and microelectronic devices generally attempt to minimize signal line length for many reasons in addition to minimizing signal intensity attenuation, and signal line length minimization is constrained by component subcomponent sizes, heat dissipation requirements, internal electrical field and radio frequency interference, device geometries, and other such factors. Therefore, designers and manufacturers of circuit boards and other microelectronic devices employing conductive traces have recognized the need for identifying and employing new, inexpensive dielectric substrate materials with significantly lower loss tangents in order to maintain signal intensity attenuation below acceptable threshold values.
One embodiment of the present invention involves substitution of air for common dielectric substrates in which striplines of circuit boards and microelectronic devices are embedded. Air has an extremely low loss tangent, which, in turn, greatly decreases the attenuation coefficient xcex1D for traces surrounded by air relative to traces surrounded by solid dielectric substrate materials. The traces are initially embedded within a filament-containing substrate material during manufacture. A matrix portion of the substrate material is then removed by melting, sublimation, chemical dissolution, or other removal technique to leave the traces suspended between conductive reference planes by the filaments. The conductive reference planes are maintained in rigid, parallel displacement from one another by any of various support or framing structures to which the conductive reference planes are affixed or mounted prior to removal of the matrix portion of the filament-containing substrate.